Papaya (Carica papaya L.)(“CP”) is an important fruit crop grown widely in tropical and subtropical lowland areas. Brazil, India, and Mexico are the largest producers of papaya. Hawaii is the largest producer of papaya in the United States. About 66% of the total production of Hawaiian papaya is exported, primarily to mainland United States and Japan (Gonsalves, D., “Control of Papaya Ringspot Virus in Papaya: A Case Study,” Ann. Rev. Phytopathol. 36:415–37 (1998)). Unfortunately, the fruit is fragile, a characteristic limiting large-scale exportation of mature papaya to countries in temperate regions. To minimize this problem, the current practice is to collect fruits for exportation in very precocious phases of maturation with the consequence of adulteration of the organoleptic characteristics of this fruit. This early harvest of fruit, designed to avoid damage in subsequent handling, can result in a failure to develop optimum fruit flavor and color.
Another tactic employed to slow the ripening process in-transit is to ship and store papaya at cold temperatures. This practice ultimately results in significant fruit damage also, as papaya fruit is susceptible to chilling injury, with critical temperature ranging between 10–15° C. In papaya, the symptoms of chilling injury are more evident upon returning the fruit to higher ripening temperatures, which results in excessive softening and the associated enhancement of pathogen susceptibility (Chan et al., “Electrolyte Leakage and Ethylene Production Induced by Chilling Injury of Papayas,” Hort. Science 20:1070–1072 (1985); Lyons et al., “Chilling Injury,” in Weichmann, ed., Postharvest Physiology of Vegetables, New York: Marcell Dekker Inc., pp. 305–326, (1987)). In an effort to solve the problems associated with long-distance shipping of fruit, researchers have concentrated on unraveling the role of enzymes involved in the ripening process. Three enzymes that have surfaced as vital for fruit ripening are pectinmethylesterase (“PME”), β-glucuronidase (“β-Gal”), and the polygalacturonase (“PG”) family.
PME is a pectolytic enzyme which has been implicated in fruit ripening (Bacic et al., “Structure and Function of Plant Cell Walls. In the Biochemistry of Plant: A Comprehensive Treatise,” ed. J. Preiss, 14:297–371, New York: Academic (1988)). This cell wall metabolizing enzyme is responsible for the demethylation of galacturonic acid residues in high molecular weight pectin, each methyl group being converted to a proton and methanol (Hall et al., “Molecular Characterization of cDNA Clones Representing Pectin Esterase Isozymes from Tomato,” Plant Mol. Biol. 25(2):313–318 (1994)). PME activity has been reported to increase during the development of banana (Brady, “The Pectinesterase of Pulp Banana Fruit,” Aust. J. Plant Physiol. 3:163–172 (1976)), apple (Knee M., “Metabolism of Polygalacturonase in Apple Fruit Cortical Tissue During Ripening,” Phytochemistry 17:1262–1264 (1979)), avocado (Awad et al., “Postharvest Variation in Cellulase, Polygalacturonase and Pectin Methylesterase in Avocado (Persea americana) Fruit in Relation to Respiration and Ethylene  Production,” Plant Physiol. 64:306–308 (1979)), and papaya (Paull et al., “Postharvest Variation in Cell Wall Degrading Enzymes of Papaya (Carica papaya) During Ripening,” Plant Physiol. 72:382–385 (1983)). The exact role of PME in fruit development and ripening is yet to be determined. However, it has been hypothesized that de-esterification of pectin by PME and further depolymerization by PG are involved in fruit softening. This hypothesis is based on the observation that demethylation of pectin by PME causes a several-fold increase in cell wall solubilization by PG (Pressey et al., “Solubilization of Cell Wall by Tomato Polygalacturonase Effects of Pectinesterase,” J. Food Biochem. 6:57–74 (1982)).
A wide range of enzymes is known to catalyze aspects of pectin modification and disassembly. Among those best characterized are exo- and endo-polygalacturonases (“PGs”), which are implicated in the disassembly of pectin that accompanies many stage of plant development, in particular those requiring cell separation. Although being clear that PG participates in a wide range of developmental processes, the majority of research has been focused on its role in fruit ripening.
PG-dependent disassembly has been most extensively studied in ripening tomatoes. Following the experiences of suppression of PG gene expression in wild type tomato and on the ectopic expression of PG in the ripening impaired pleiotropic mutant ripening inhibitor (“rin”), it has been considered that PG-mediated pectin depolymerization is not necessary for normal ripening and softening (Sheehy et al., “Reduction of Polygalacturonase Activity in Tomato Fruit by Antisense RNA,” Proc. Natl. Acad. Sci. USA 85:8805–8809 (1988); Smith et al., “Antisense RNA Inhibition of Polygalacturonase Gene Expression in Transgenic Tomatoes,” Nature 334:724–726 (1988); Giovannoni et al., “Expression of a Chimeric Polygalacturonase Gene in Transgenic Rin (Ripening Inhibitor) Tomato Fruit Results in Polyuronide Degradation But Not Fruit Softening,” Plant Cell 1:53–63 (1989)). Research performed with transgenic sense and antisense tomatoes suggests that PG-mediated pectin disassembly does not contribute to early fruit ripening but contributes to tissue deterioration in the late stages of fruit ripening (Hadfield et al., “Polygalacturonase Gene Expression in Ripe Melon Fruit Supports a Role for Polygalacturonase in Ripening-Associated Pectin Disassembly,” Plant Physiol. 117: 363–373 (1998)). Analysis of cell walls from transgenic fruits with altered levels of PG activity led to the conclusion that pectin depolymerization and pectin solubilization are due to distinct enzymatic determinants (Hadfield et al., “Polygalacturonase: Many Genes in Search of a Function,” Plant Physiol. 117:337–343 (1998)). According to the same authors, pectin solubilization is primarily due to the action of PG. The fact that pectins in PG-complemented rin fruit are both solubilized and depolymerized accounts for the conclusion that PG activity is necessary and sufficient for pectin depolymerization, but it may be one of multiple, redundant pectin-solubilizing activities (Hadfield et al., “Polygalacturonase: Many Genes in Search of a Function,” Plant Physiol. 117:337–343 (1998)).
In papaya, the gradual firmness loss of fruit is associated with a discernible, although very limited, increased in PG activity (Ali et al., “The Biochemical Basis of Accelerated Softening in Papaya Following Storage at Low Temperature,” Acta Horticulture 343 (1993)). In contrast, other fruits such as strawberry (Fragaria ananassa) (Huber, “Strawberry Fruit Softening: The Potential Roles of Polyuronides and Hemicelluloses,” J. Food Sci. 49:1310–1315 (1984)), melon (Cucumis melo) (McCollum et al., “Modification of Polyuronides and Hemicelluloses During Muslanelon Fruit Softening,” Physiol. P1. 76:303–308 (1989)), and persimmon (Diospyrus kaki) (Cutillas-Iturralde et al., “Metabolism of Cell Wall Polysaccharides from Persimmon Fruit: Solubilization During Fruit Ripening Occurs in Apparent Absence of Polygalacturonase Activity,” Physiol. Plant. 89:369–375 (1993)) have been reported as lacking endo-PG activity. Recently, it was demonstrated that PG mRNA accumulation can occur at late stages of ripening at levels much lower than those observed in ripening tomato, only detectable by using very accurate methods (Wu et al., “Endopolygalacturonase in Apples (Malus domestica) and its Expression During Fruit Ripening,” Plant Physiol. 102:219–225 (1993)). It has also been reported that of three genes encoding melon PGs, one of those (MPG1) encodes an endo-PG with the potential to depolymerize melon fruit cell wall pectin (Hadfield et al., “Polygalacturonase Gene Expression in Ripe Melon Fruit Supports a Role for Polygalacturonase in Ripening-Associated Pectin Disassembly,” Plant Physiol. 117: 363–373 (1998)). It is therefore possible that in some fruits the disassembly of pectins in late stages of ripening is PG dependent, even in fruits with very low levels of PG activity (Hadfield et al., “Polygalacturonase: Many Genes in Search of a Function,” Plant Physiol. 117:337–343 (1998)).
Another enzyme that has been implicated in fruit ripening is β-Gal, an enzyme involved in cell wall softening and known to exist in three isoforms (β-Gal I, β-Gal II, and β-Gal III). In “β-Galactosidases in Ripening Tomatoes,” Plant Physiol. 71:132–135 (1983), Pressey et al., reported on the increase of activity of one of the three β-galactosidases isozymes during tomato ripening, suggesting that these isozymes may play a role on degradation of cell wall galactan, which may account for the involvement of β-Gal in fruit softening. The involvement of β-Gal in tomato fruit ripening has been confirmed (Watkins et al., “Activities of Polygalacturonase α-D Mannosidase and α-D and β-D Galactosidases in Ripening Tomato,” HortScience 23: 192–94 (1988)). More recently, the increase of β-Gal during ripening of kiwi fruit (Wegrzyn et al., “Pectinesterase, Polygalacturonase and β-Galactosidase During Softening of Ethylene-Treated Kiwi Fruit,” HortScience 27:900–902 (1992)), mango and papaya (Lazan et al., “Cell Wall Hydrolases and Their Potential in the Manipulation of Ripening of Tropical Fruits,” Asean Food J. 8:47–53 (1993)), avocado (De Veau et al., “Degradation and Solubilization of Pectin by β-Galactosidases Purified from Avocado Mesocarp,” Physio. Plant 87:279–285 (1993)), and coffee (Golden et al., “β-Galactosidase from Coffea arabica and its Role in Fruit Ripening,” Phytochemistry 34:355–360 (1993)) have been reported. In apples, the loss of fruit firmness during ripening has been associated with increased activity of β-galactosidase and a decrease in the Gal content of the cell wall (Bartley, “β-Galactosidase Activity in Ripening Apples,” Phytochemistry 13:2107–2111 (1974); Wallner, “Apple Fruit β-Galactosidase and Softening in Storage,” J. Am. Soc. Hort. Sci. 103:364 (1978)). Furthermore, Kang et al., “N-Terminal Amino Acid Sequence of Persimmon Fruit β-galactosidase,” Plant Physiol. 105:975–979 (1994) purified two isozymes (one with 34 kD and the other with 44 kD) from persimmon fruit. A characteristic feature during the ripening of papaya fruit is softening. β-galactosidase might contribute significantly to pectin and hemicellulose modification and, hence, to softening of the fruit (Lazan et al., “β-galactosidase, Polygalacturonase and Pectinesterase in Differential Softening and Cell Wall Modification During Papaya Fruit Ripening,” Physiol. Plant 95:106–112 (1995)).
According to Ali et al., “The Biochemical Basis of Accelerated Softening in Papaya Following Storage at Low Temperature,” Acta Horticulture 343 (1993), PME, PG, and the β-Gal isoforms may collectively play a significant role in the development of the chilling injury symptom of increased-susceptibility-to disease commonly observed in papaya upon returning chill-stored fruits to warmer environments.
Attempts to deliver mature, full-flavored, and unadulterated papaya fruits to the consumer by long-distance transport have concentrated thus far on largely unsuccessful measures such as early harvest and low temperature storage. What is needed is a solution which utilizes and adapts the natural maturation process of the papaya such that the fruit can tolerate the stresses of long-distance exportation.
The present invention is directed to overcoming these and other deficiencies in the art.